1. Field of Invention
The present invention relates to a field of optoelectronic technology, and more particularly to a device for measuring refractive index of medium based on optical delay technology and its method.
2. Description of Related Arts
Methods for measuring refractive index of medium can be divided in two types, i.e., the transmission type and the reflection type.
In the transmission-type method, the refractive index of medium to be measured is calculated based on the characteristic of the transmitted light which has passed through the medium to be measured. The transmission-type method mainly comprises: measuring the angle of incidence of the incident ray and the angle of refraction of the refracted ray; and calculating the refractive index according to the formula of law of refraction, also called Snell's law. The transmission-type method is simple and easy to realize. The measuring accuracy depends on accurate measurement of the angle of incidence and the angle of refraction, and the measuring device is complicated. Meanwhile, the medium to be measured is required to be the optically transparent medium, such as water, glass, and crystal. The transmission-type method is not applicable for the turbid medium, such as milk which is a liquid, because the light passing through the medium will be scattered and absorbed.
In the reflection-type method, the refractive index of medium to be measured is calculated based on the parameters of light reflection characteristic, such as reflectivity, polarization, phase and critical angle, wherein the reflection happens at the medium interface. The critical angle method is typical in the reflection-type method. According to the law of refraction, when the light passes through the optically thinner medium from the optically denser medium, the angle of refraction is larger than the angle of incidence. In addition, when the angle of incidence increases, the angle of refraction will increase. When the angle of incidence increases to a certain value, the angle of refraction will increase to 90 degrees, which means that the refracted ray propagates along the interface between the two media. The angle of incidence in this case is called critical angle. If the angle of incidence continues to increase after achieving the critical angle, the light will not propagate into the optically thinner medium. Instead, the light is completely reflected back to the optically denser medium, which is a phenomenon known as total reflection. When the refractive index of the optically denser medium does not change, the critical angle only depends on the refractive index of the optically thinner medium. The device designed based on this method is called critical angle refractometer. The measuring method has a simple principle. The measuring accuracy depends on accurate measurement of the angle. The device is complicated, but the device is widely applicable for many measuring objects, comprising non-transparent medium, translucid medium, and transparent medium, for example, metal, milk, etc.